U.S. Pat. No. 4,977,419 discloses a type of photographic film having a transparent magnetic layer that covers the entire side of the film base opposite to that of the photosensitive layer, and a type of camera that is equipped with a magnetic head for writing to and reading from the magnetic layer. The head is used to record information about each picture taken on several tracks that are located outside of the image area of the film. Examples of the information that may be recorded include the date and time of the exposure, roll and frame titles, type of illuminant used, pseudo-zoom information, and other photographic information such as shutter speed and lens aperture value.
U.S. Pat. No. 4,878,075 discloses a type of film having magnetically prerecorded information about the attributes of a specific strip of the film, such as photographic speed and number of photographic frames present. It also discloses a type of camera that reads this information from the film prior to a picture-taking event and then rewrites it, along with the kind of information relating to the picture-taking event that is described above, after the picture-taking event when the film is being wound to the next frame. The camera writes information on a track of each exposed frame indicating that the frame has been exposed ("exposed information"), and then advances the film to the next available unexposed frame to resume exposure of the roll.
In order to fully realize the potential capacity of a film having a magnetic-memory region, magnetic heads used for the above purposes are required which are capable of writing to and reading from a roll of film that has been loaded into a camera. Preferably, these magnetic heads should possess one track-forming portion that has the capability to both write information to and read information from the magnetic memory portion of the film, and at least one additional track-forming portion that has only write capability. In addition, the track-forming portions should be sufficiently small so that any increase in the size of the camera due to the presence of the magnetic heads is minimized.
The design of conventionally known wirewound inductive magnetic heads, such as those described above, is made difficult by the requirement that they read information recorded on film. In general, the following expression holds for the signal output from such a magnetic head: EQU V=k.times.D.times.N.times.W.times.v
in which:
k is a constant; PA1 D is a constant proportional to the dispersion of the magnetic layer; PA1 N is the number of turns in the sensing coil that is wound on the core; PA1 W is the width of the core at the location that defines the magnetic track; and PA1 v is the speed at which the magnetic media passes by the magnetic head. PA1 the core has a major dimension extending in a direction which is parallel to the plane of the film. PA1 the first track-forming portion is arranged further from a photographic frame of the film than the second track-forming portion. PA1 one of the track-forming portions for reading information from the magnetic-memory region of the film is arranged farther from a photographic frame of the film than the other track-forming portions.
Each of the above parameters are restricted in the following ways: First, in a camera operating with magnetically treated film, the magnetic layer is typically dispersed along the entire film base on the side opposite to the photosensitive side of the film, such as the one referred to in the '419 patent. In order not to degrade the image quality of the film, this layer is usually transparent, therefore, it should preferably contain a very dilute dispersion of magnetic particles. This produces a value of D which is small.
The widths of the magnetic tracks should be as small as possible, since they are preferably located in an area which is outside of the frame area of the film (and is a region preferably minimized in order to minimize the width of the film) while maximizing the amount of information stored by providing as many tracks as possible. Therefore, W is small.
The film typically is transported at low speeds, on the order of about 100 mm/second, due to limitations on the power available from battery packs internal to the camera. Therefore, v is also small.
Furthermore, the magnetic head of necessity is typically located near to sources of electromagnetic interference ("noise") such as the film winding motor. Therefore, the signal-to-noise ratio produced by the magnetic head is low, which results in a poor reliability of reading information from the film in the camera. Under these conditions, it is apparent that a head with a large number of turns is required to produce an output level sufficient to provide an adequate signal to noise ratio. Increasing the number of turns in a conventional head structure, as shown in FIG. 1 can only be accomplished by increasing the dimension "H". Such an increase is inconsistent with the requirement of keeping the camera size to a minimum.